Beta cells (“β cells”) are a type of islet cell found in the pancreas that produce and secrete the hormone insulin. Insulin controls levels of glucose in blood. Type 1 diabetes mellitus (T1DM) is an autoimmune disease characterized by the selective destruction of pancreatic β cells, while type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by insulin resistance and a loss of β cell function and mass. β cell apoptosis is central to disease progression in both T1DM and T2DM. Preventing β cell apoptosis is a key factor in the successful outcome of islet transplantation as a treatment for T1DM. Proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α), interleukin-β (IL-1β), interferon-γ (IFN-γ), oxygen free radicals, and nitric oxide are implicated in promoting β cell death (1, 2). Although the role of TNF-α as an effector remains ambiguous (3-8), a combination of IFN-γ and TNF-α synergistically induces β cell apoptosis, and is a key factor in the development of autoimmune diabetes (9). In T2DM, TNF-α is a key mediator in insulin resistance associated with obesity (10-12). TNF-α not only induces insulin resistance in insulin-sensitive tissues, such as adipose tissue and skeletal muscle (13-15), but also decreases glucose stimulated insulin secretion (GSIS) in β cells (14). These findings suggest that TNF-α mediates dysfunction and/or destruction of β cells in both T1DM and T2DM.
Inflammation contributes to β cell destruction, prolonged suppression of β cell function, inhibition of β cell regeneration, and even peripheral insulin resistance (34). Cytokine induced cell death has been shown to contribute to β cell apoptosis through an intrinsic pathway (21, 22). The proinflammatory cytokine TNF-α has been shown to play an important role in the pathogenesis of T1DM as well as T2DM. Human islets express a high level of tumor necrosis factor receptor superfamily (TNFRSF) 1A. Since mRNAs of ligands for TNFRSF1A are constitutively expressed in peripheral blood leukocytes and are induced at a high levels by stimulation (35, 36), receptors for TNF-α on islet cells would play a significant role in inflammation.
TNF-α can induce both apoptotic and anti-apoptotic signals regulated by the activation of NFκB (16-18). TNF-α mediated apoptosis through the TNF receptor associated death domain (TRADD), where specific ligand-receptor binding leads first to activation of Caspase-8 and then to activation of Caspase-3 via an extrinsic pathway (19). Alternatively, apoptosis through intracellular or intrinsic pathways is caused by DNA damage, hypoxia, nutrient deprivation, or reactive oxygen species (ROS) function via the mitochondrial pathway and tightly modulated by the Bcl-2 proteins (also called mitochondrial pathway or Bcl-2 regulated pathway). These triggering factors lead to mitochondrial membrane permeability and a subsequent release of cytochrome c from the intermembranous space, followed by the activation of Caspase-9, which in turn activates Caspase-3 (20). Bcl-2 homology 3 (BH3) only protein, Bid (BH3 interacting domain death agonist), is shown to be essential for death receptor-induced apoptosis of pancreatic β cells in mice (21). IL-1-β, IFN-γ and/or TNF-α induce cell death in rat islets through the intrinsic pathway by dephosphorylation of the BH3 only protein, Bad (BCL2-associated agonist of cell death) (22). However, interactions between extrinsic and intrinsic pathways in cytokine induced cell death of human pancreatic β cells remains unclear.
PUMA (p53 upregulated modulator of apoptosis) is one of the most potent killers among the BH3-only subgroup of Bcl-2 member protein (23, 24). It is induced by p53 following DNA damage, irradiation or chemotherapeutic drugs (25). PUMA/BBC3 (Bcl-2 binding component 3) can be directly activated through p53 responsive elements in its promoter region (26) or independently of p53 by other transcription factors initiating apoptotic responses, including growth factor/cytokine deprivation (27), endoplasmic reticulum stress (28), and ischemia reperfusion (29, 30). PUMA is also activated by the p65 component of NF-κB through a κB site in the PUMA promoter in response to TNF-α (31). It would be a significant improvement in the art to understand and measure the role of PUMA in islet cell death, particularly in β cell specific death, which has heretofore been unknown. It would also be a significant improvement to use such understanding to develop PUMA as a biomarker and to make and use PUMA-based therapies for controlling islet cell apoptosis.